This invention relates to fiber optic cables and the structure for reinforcing the tensile and compressive strength characteristics of the optical fibers contained within the fiber optic cables. Specifically, the invention is directed toward an improved structure for use in low fiber-count cable construction.
It is well known that small, light-transmitting optical fibers are mechanically fragile and exhibit poor strength characteristics. Typically, fiber optic cables which are intended for use in outside environments are designed for long-haul applications and such cables, therefore, have a medium to high fiber-count. Such long-haul cables are relatively rigid and have a relatively large bending radius as a result of the high fiber-count and resulting cable structure. These long-haul type cables are undesirable for use in short-haul applications where local distribution type networks typically require fiber optic cables having great flexibility and low fiber counts. A requirement of such short-haul local distribution fiber optic cables is that the cable be greatly flexible and have a small bending radius so that it is useable in a variety of environments.
The challenge of providing a low count fiber optic cable having desirable flexibility, small bending radius, as well as appropriate tensile and compressive strength characteristics has produced a large variety of cable structures. For instance, U.S. Pat No. 4,723,831 provides a cable construction consisting of optical fibers centrally located within an overwrap. An outer jacket of plastic material such as polyvinylchloride encloses the overwrapped optical fibers and has glass strength members imbedded within the jacket. The glass optical fibers are treated with a coupling agent such as a urethane so that they are coupled with the jacket to have a desired pullout strength of more than 40 pounds per square inch. U.S. Pat. No. Re. 32,436 discloses the use of a optically transparent silica core that is protected by fibers positioned parallel to the core which are used as strength members in tension. A plastic jacket is extruded around the combination of core and fibers. The reinforcement fibers are designed to have an elastic modulus of at least 10 million psi. Another short-haul design, U.S. Pat. No. 4,241,979 provides a cable structure in which strength members (steel wire or non-metallic fibers) are bedded in a bedding compound of a thin layer of spun bonded polyester. An outer jacket of polyethylene is extruded over the bedding layer and the strength members. The thickness and compliance of the bedding layer determines the amount of coupling between the plastic jacket and the strength members.
It is known that there are numerous designs of fiber optic cables which are all directed, in some way, to creating specific tensile and compressive strength in the cable while providing a desired flexibility and low bending radius. The design approaches have centered on two successful alternatives. The first includes at least one buffer tube containing the optical fibers positioned longitudinally or helically and surrounded by strength members and sheathing. The unstranded loose-tube cable provides for some flexibility and separates tensile and compressive forces from the optical fiber. In other designs a buffer tube filled with a compound to prevent the intrusion of water and other liquids into the cable is surrounded by strength members and sheathing. These designs have been modified in other applications to include a steel armour coupling surrounding the buffer tube and strength members to prevent destruction of the fiber optic cable by rodents.
A secondary focus in the design of fiber optic cables is the appropriate provision of anti-buckling characteristics into the cable. Anti-buckling is defined as the resistance provided by a material to cable shrinkage. Generally, the anti-buckling can be estimated by the low temperature modulus of the cable materials times the change in length at low temperature. The amount of anti-buckling required must usually control the cable shrinkage from between 0.2% to 0.8%. Anti-buckling is provided through emphasis on materials used in jackets, overcore concepts and circumferentially or helically applied anti-buckling materials.
Finally, while providing the necessary tensile and compressive strength as well as desired anti-buckling characteristics, the cable design must provide sufficient cable flexibility to provide ease of usage by technicians when installing the cable. Expectations generally require that a cable be capable of withstanding bends within a radius of 10 times the cable diameter. This flexing, crushing and twisting of the cable is a very real part of the cable environment and, therefore, must be accounted for in the cable design.
There is a continued need for an inexpensive, simple optical fiber cable construction which offers anti-buckling, tensile, and compressive strength in the cable member while maintaining cable flexibility and other desirable characteristics.